1. Field of the Invention
The present invention relates to a resin joined body, and more particularly to a resin joined body formed by fuse bonding by using a laser beam.
2. Discussion of the Background
Conventionally, there are broadly known various methods of fuse bonding resin members together as a method of joining members together. Among these methods, many methods include overlapping resin members made of a resin material together with the overlapped resin members held in surface-to-surface contact with each other and irradiating a laser beam onto the overlapped portion to fuse the surfaces of the resin material, thereby forming a fuse bonded portion.
For forming a resin joined body having the surfaces of the resin members fuse bonded together, infrared laser or near infrared laser is usually used.
Also, for forming a resin joined body, a substance having an absorption peak in the infrared portion or near infrared portion (hereinafter also referred to absorbent), such as carbon black and porphyrin dye, is disposed in the surface-to-surface contact portion of the resin members. This absorbent remains in this joined portion even after the formation of the resin joined body. Since the absorbent as mentioned above contains dye or the like, an absorbent remaining portion has a deteriorated aesthetic appearance having such as a lowered light transmittance than the other portion.
It is conceivable to reduce the amount of the absorbent in order to prevent deterioration of the aesthetic appearance such as lowering of the light transmittance, while increasing the irradiation energy of a laser beam. However, this may pose a problem. That is, when forming this conventional resin joined body, a laser beam is usually irradiated from a rear side of one resin member having its surface contacting the opposite resin member to the surface-to-surface contact portion. This laser beam passes through the resin members so that the laser beam is absorbed by the absorbent. In addition, although not to such an extent as the absorbent does, the resin members themselves also absorb the laser beam to some extent, and therefore the increase in irradiation energy of the laser beam causes heat developing on this rear side of this resin member. In some situations, the rear side may be fused.
[Patent Document 1 ]describes a method using a focused laser beam, as a method of forming a fuse bonded portion in a surface-to-surface contact portion by using such laser beam. In a method using this focused laser beam, a laser beam, which passes from the rear side towards the surface side, of a resin member, is converged so that the energy of the laser beam can be focused near the focal point of the focused laser beam, and therefore it is possible to form a fuse bonded portion while suppressing heat developing on the rear side of the resin member and reducing the number of times the absorbent is used.
However, merely the use of the focused laser beam makes it difficult to generate sufficient heat energy in the surface-to-surface contact portion of the resin members while at the same time satisfactorily suppressing the use of the absorbent. That is, a conventional resin joined body that has a fuse bonded portion formed therein by the irradiation of a laser beam has a problem that the aesthetic appearance of the joined portion cannot be satisfactorily improved.
[Patent Document 1] Japanese Patent Application Laid-open No. 2004-195829
It is an object of the present invention to provide a resin joined body that has a joined portion with a satisfactorily improved aesthetic appearance.